Fejl i sql class - cannot acces private property

Hej eksperter. Jeg er igang med at lave en mysql class, men ved test får jeg fejlen:

Fatal error: Cannot access protected property DB_MysqlStatement::$result in C:\Server\www\cache\lib\DB_Mysql.php on line 52

Umiddelbart kan jeg ikke selv se hvor fejlen ligger. Jeg er relativ ny i OO Programmering.


---------test.php--------------------
<?php
require "lib/DB_Mysql.php";
$dbh = new DB_Mysql("brugernavn","kodeord","localhost","database");
$query = "SELECT * FROM users WHERE user_id = '1'";
$stmt = $dbh->execute($query);
?>
---------DB_Mysql.php-----------------
<?php
class DB_Mysql {

    protected $user;
    protected $pass;
    protected $dbhost;
    protected $dbname;
    protected $dbh;        // Database connection handle
   
    public function __construct($user, $pass, $dbhost, $dbname) {
        $this->user = $user;
        $this->pass = $pass;
        $this->dbhost = $dbhost;
        $this->dbname = $dbname;
    }
   
    protected function connect() {
        $this->dbh = mysql_pconnect($this->dbhost, $this->user, $this->pass);
        if(!is_resource($this->dbh)) {
            throw new Exception;
        }
        if(!mysql_select_db($this->dbname, $this->dbh)) {
            throw new Exception;
        }
    }
   
    public function execute($query) {
        if(!$this->dbh) {
            $this->connect();
        }
        $ret = mysql_query($query, $this->dbh);
        if(!$ret) {
            throw new Exception;
        }
        else if(!is_resource($ret)) {
            return true;
        } else {
            $stmt = new DB_MysqlStatement($this->dbh, $query);
            $stmt->result = $ret;
            return $stmt;
        }
    }
}

class DB_MysqlStatement {
   
    protected $result;
    public $query;
    protected $dbh;

    public function __construct($dbh, $query) {
        $this->query = $query;
        $this->dbh = $dbh;
        if(!is_resource($dbh)) {
            throw new Exception("Din database connection er ikke gyldig");
        }
    }
   
    public function fetch_row() {
        if(!$this->result) {
            throw new Exception("Din query-forespørgsel blev ikke fuldført");
        }
        return mysql_fetch_row($this->result);
    }
   
    public function fetch_assoc() {
        return mysql_fetch_assoc($this->result);
    }

    public function fetchall_assoc() {
        $retval = array();
        while($row = $this->fetch_assoc()) {
        $retval[] = $row;
        }
        return $retval;
    }
}
?>